* This is not direct solution for shared user-upload media, but can be a handy option for cross-network static files, and might provide you with an option of sorts.

When using multisite with sub-domains and domain mapping all mapped sites can easily share static files. As new sites are mapped to the primary domain, you can simply add a folder to the root level of your server and acts as a relative folder for every mapped domain. Kinda.

For Example

New Folder = img
Create Folder @ yourwpmsdomain.com/img

Then all domains (mapped and install) lead to /img
yourwpmsdomain.com/img
mappeddomain1.com/img
mappeddomain2.com/img
allmappeddomains.com/img